Not sure it really matters, but I would put the mulchers up top. I wouldn't really recommend running double blades with a high lift and mulching blade combo though. It's going to sound like a freaking helicopter and throw the grass 3 to 4 mower widths and put a lot of strain on the belt and PTO. Your deck will likely quickly pack with grass too. If you're intent on using double blades I'd recommend running a combo of low lift blades and either a wavy mulcher or Gator-type mulching blades. If you do run double blades, they should be precision balanced and you should make sure that your bolts are long enough. Note that you'll be cutting lower so you'll have to raise your deck up a notch.
Instead of double blades I'd recommend trying a single set of Gator G5 or G6 blades. They reduce clipping size and still have enough lift to discharge well and provide a decent cut. They're wider than standard blades so they can get more of a "sail" or air foil on them. Basically, they're high-lift mulching blades. The G5 and G6 are the same design but the G6 is made of thicker steel for increased durability and are very heavy. Both are pretty heavy in fact and require a machine with sufficient engine power.